i

ORGANICALLY MODIFIED SILICA

NANOSTRUCTURES BASED FUNCTIONAL

COATINGS FOR PRACTICAL APPLICATIONS

A THESIS SUBMITTED TO

OF THE GRADUATE SCHOOL OF ENGINEERING AND SCIENCE

OF BILKENT UNIVERSITY

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR

THE DEGREE OF

MASTER OF SCIENCE

IN

MATERIALS SCIENCE AND NANOTECHNOLOGY

By

Urandelger Tuvshindorj

August, 2015

ii

ORGANICALLY MODIFIED SILICA NANOSTRUCTURES BASED

FUNCTIONAL COATINGS FOR PRACTICAL APPLICATIONS

By Urandelger Tuvshindorj

August, 2015

We certify that we have read this thesis and that in our opinion it is fully adequate, in

scope and in quality, as a thesis of the degree of Master of Science.

Prof. Dr. Mehmet Bayındır (Advisor)

Assoc. Prof. Dr. Hatice Duran

Assist. Prof. Dr. H. Tarik Baytekin

Approved for the Graduate School of Engineering and Science:

Prof. Dr. Levent Onural

Director of the Graduate School

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ABSTRACT

ORGANICALLY MODIFIED SILICA NANOSTRUCTURES BASED

FUNCTIONAL COATINGS FOR PRACTICAL APPLICATIONS

Urandelger Tuvshindorj

M.S. in Materials Science and Nanotechnology

Supervisor: Prof. Dr. Mehmet Bayındır

August, 2015

In the past decades, the fabrication of superhydrophobic surfaces have received

considerable attention due to the variety of potential applications ranging from biology

to industry. Although significant progress has been made in their fabrication and

design, there is still need to solve some problems in real-life use of these coatings, such

as low stability against external pressure, lack of long term robustness, challenges in

presice control over the degree of wettability and the need for facile fabrication

methods. In this context, this thesis seeks simple solutions for mentioned problems

based on organically modified silica superhyrophobic coatings.

First, we investigate the stability of the Cassie state of wetting in transparent

superhydrophobic coatings by comparing a single-layer micro-porous coating with a

double-layer micro/nanoporous coating. The stability of the Cassie state in coatings

were investigated with droplet compression and evaporation experiments, where

external pressures as high as a few thousand Pa are generated at the interface. A droplet

on a microporous coating gradually transformed to the Wenzel state with increasing

pressure. The resistance of the micro/nano-porous surfaces against Wenzel transition

during the experiments were higher than microporous single-layer coating and even

higher than leaves of Lotus; prevalent natural superhydrophobic surface. Then, we

reported a facile method for the preparation hydrophilic patterns on the

superhydrophobic ormosil surfaces. On the defined areas of the superhydrophobic

ormosil coatings, wetted micropatterns were produced using Ultraviolet/Ozone

iv

(UV/O) treatment which modifies the surface chemistry from hydrophobic to

hydrophilic without changing the surface morphology. The degree of wettability of the

patterns can be precisely controlled depending on the UV/O exposure duration and

extremely wetted spots with water contact angle (WCA) of nearly 0º can be obtained.

The ormosil coatings and modified surfaces preserve their wettability for months at

room conditions. Furthermore, we demonstrated selective and controlled adsorption of

protein and adhesion of bacteria on the superhydrophilic patterns which could be

potentially used for biological applications.

Keywords: Superhydrophobic, Cassie state stability, Self-cleaning, Wettability,

Droplet evaporation, Organically modified silica, Superhydrophilic, UV/O treatment,

Controlled wetting, Biomolecular adsorption, Cell pattern

v

ÖZET

ORGANİK OLARAK MODİFİYE EDİLMİŞ NANOYAPİLİ

MALZEMEDEN FONKSİYONEL YÜZEY ELDE EDİLMESİ VE

ONLARIN PRAKTIKAL UYGULAMALI İÇİN ÇÖZÜMLERİ

Urandelger Tuvshindorj

Malzeme Bilimi ve Nanoteknoloji, Doktora

Tez Yöneticisi: Prof. Dr. Mehmet Bayındır

Ağustos, 2015

Son yıllarda, süperhidrofobik yüzeyler biyolojiden endüstriye kadar potansiyel

geniş kullanım alanlardan dolayı çok fazla dikkat çekmektedir. Her ne kadar bu

yüzeylerin tasarım ve üretiminde kayda değer gelişme kaydedilmiş olsa da, hala bu

süperhidrofobik kaplamaların günlük hayatta kullanımının önünde dış basınca karşı

dayanıksızlık, özelliklerini uzun süre koruyamama, yüzeyin ıslanma özelliğinin tam

olarak kontrol edilememesi ve üretim yöntemlerinin pahalı olması gibi problemler

vardır. Bu kapsamda, bu tez bahsedilen problemlerin çözümü için organik olarak

modifiye edilmiş silika yüzeyler ile basit çözümler aramaktadır.

Öncelikle, şeffaf süperhidrofobik kaplamalarda Cassie durumunun basınca

dayanıklılığı, tek katmanlı mikro-gözenekli kaplamaların, çift katmanlı mikro/nano-

gözenekli kaplamalar ile kıyaslanması ile incelenmiştir. Kaplamaların dayanıklılığı,

damlacık-kaplama ara yüzünde binlerce Paskal basınç oluşturulan damlacık sıkıştırma

ve buharlaştırma deneyleri ile incelenmiştir. Tek katmanlı mikro-gözenekli kaplamalar

artan basınç ile aşama aşama Wenzel durumuna geçmiştir. Mikro/nano-gözenekli

kaplamalar ise Wenzel geçişi öncesinde çok daha yüksek basınçlara dayanmıştır. Bu

çift katmanlı kaplamaların basınca karşı dayanıklılığı, doğal süperhidrofobik yüzeyler

için iyi bilinen örneklerden olan Lotus bitkisi yapraklarından dahi daha iyi düzeydedir.

Ormosil kaplamalar aynı zamanda basit yöntemlerde çok miktarlarda

üretilebilmektedir ve süperhidrofobik özelliklerini oda koşullarında aylarca

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koruyabilmektedir. Bir sonraki aşamada süperhidrofobik ormosil yüzeyler üzerinde

hidrofilik yapılar oluşturulabilmesi için basit bir yöntem geliştirilmiştir.

Süperhidrofobik yüzeylerin önceden belirlenen alanlarında Morötesi/Ozon (UV/O)

uygulanarak bu alanların yüzey morfolojisinde herhangi bir değişik olmadan hidrofilik

özelliğe sahip olması sağlanmıştır. Bu yüzeylerin ne kadar ıslanabilir olduğu UV/O

uygulamasının süresine göre tam olarak istendiği düzeyde ayarlanabilmektedir ve su

temas açıları neredeyse 0º olan ıslanabilir noktalar elde edilebilmektedir. Ayrıca bu

ıslanabilir noktalar ile yüzeye seçici ve kontrollü biçimde protein ve bakterilerin

bağlanabildiği gösterilmiştir ki bu süperhidrofobik yüzeylerin biyolojik uygulamaları

için önem taşımaktadır.

Anahtar kelimeler: Süperhidrofobik, Cassie durumu dayanıklılığı, Kendi kendini

temizleyen yüzeyler, Islanabilirlik, Buharlaştırma, Organic olarak modifiye edilmiş

silika, Süperhidrofilik, UV/O uygulaması, Kontrollü ıslanabilirlik, Biyomolekül

adsorpsiyonu, Hücre paternleri

vii

Acknowledgement

First and foremost, I would like to thank my advisor Prof. Mehmet Bayındır for his

continuous guidance throughout my studies. He always encouraged and supported me

with the research projects.

I would like to especially thank Dr. Adem Yildirim who was like my second advisor

I have learned a lot from him during these times.

Also, I would like to thank the people who contributed to this thesis; Prof. Çağlar

Elbuken, Pınar Beyazkılıç, Fahri Emre Öztürk. Without their helps this thesis would

be incomplete.

I would like to thank all members of Bayındır group and the friends in UNAM. It

will be a long list if I thank all of them one by one, but I would like to thank some of

them particularly. To my best friends; Abubakar, Emre and Yunusa; it is always fun

to work with them. Also, I would like to thank all the staff and engineers of UNAM,

for their support and helps.

I would like to express my gratitude to The Scientific and Technological Research

Council of Turkey, TÜBİTAK, for the M.S. Scholarship. This work is supported by

TÜBİTAK under the Project No. 111T696.

Last but not least, I would like thank my family who always believed in me and

supported me.

viii

Dedicated to my family; my mother, my sister,

my grandparents, my father, my uncles, Saruul and my SF.

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Contents

Chapter 1 .................................................................................................................. 16

Introduction .............................................................................................................. 16

1.1 Wetting Phenomenon ............................................................................... 18

1.2 Superhydrophilicly Patterned Superhydrophobic Surfaces ..................... 20

1.3 Organically Modified Silica Surfaces ...................................................... 22

Chapter 2 .................................................................................................................. 24

Transparent Superhydrophobic Coatings with Robust Cassie State of

Wetting…………. ..................................................................................................... 24

2.1 Introduction ................................................................................................. 24

2.2 Experimental Section .................................................................................. 25

2.3 Preparation and Characterization of Ormosil Coatings ............................... 27

2.4 Compression Experiments ........................................................................... 30

2.5 Evaporation Experiments ............................................................................ 36

Chapter 3 .................................................................................................................. 43

Patterning Superhydrophobic Ormosil Surfaces for Controlled Wetting and

Bioadhesion ............................................................................................................... 43

3.1 Introduction ................................................................................................. 43

3.2 Experimental Section .................................................................................. 45

3.3 UV/Ozone Treated and Untreated Surfaces: Preparation and Characterization

................................................................................................................................ 46

3.4 Superhydrophilicly Patterned Ormosil Surfaces ......................................... 51

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3.5 Controlled protein adsorption and bacteria adhesion .................................. 56

Chapter 4 .................................................................................................................. 60

Conclusions ............................................................................................................... 60

Bibliography ............................................................................................................. 62

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List of Figures

Figure 1.1: Schematic view of water contact angle and Young’s equation. .............. 18

Figure 1.2: Shematic of water droplet on rough surfaces. On the right Wenzel model

and on the left Cassie-Baxter model are shown. Adopted by Ref [1] ........................ 19

Figure 1.3: Representation of 4 wettability regimes. (i) hydrophobic surface

90°<WCA<150° (ii) superhydrophobic surface WCA>150° (iii) hydrophilic surface

10°<WCA<90° and (iv) superhydrophilic surface WCA<10° respectively. Adopted by

Ref [49] ...................................................................................................................... 20

Figure 1.4: Desert beetle and its ‘functional back’. (a) Adult female desert beetle (b)

hydrophilic non-waxy region of the back and (c) Scanning electron micrograph of the

textured surface of the depressed areas. Adopted by Ref [50] ................................... 21

Figure 1.5: Example of the applications of superhydrophobic/superhydophilic

micropatterned surfaces. (a) Surface tension confined microchannels: dye gradient.

Adopted by Ref [51] (b) close-positioned order droplet arrays. Adopted by Ref [52]

(c) Fluorescent cells cultured on a patterned substrate for 48 h. Adopted by Ref [53]

.................................................................................................................................... 21

Figure 1.6: Sol-gel reaction mechanisms of ormosil. (a). Hydrolyzation of the

organosilane monomer. (b). Condensation hydrolyzed monomers ........................... 22

Figure 1.7: Functional surfaces prepared from ormosil. Photographs of (a)

Superhydrophobic thin film on glass substrate with water contact angle 178°. Adopted

from Ref [17] (b) Bent nanoporous film coated polymer substrate (PEI). (c)

Nanoporous ormosil partially coated glass slide. Coated parts reflect less light

indicating anti-reflective property. Adopted from Ref [61] ....................................... 23

Figure 2.1: SEM images of ormosil coatings. (a-c) High magnification SEM images

of MC, NC, and MNC, respectively. The glass surface exposed through the openings

xii

in MC is outlined in orange. In the MNC, the bottom NC is outlined in orange. Low

magnification SEM images of the coatings. (d-f) MC, NC and MNC. Arrows indicate

the underlying glass substrate in (d) and NC in (f). ................................................... 28

Figure 2.2: (a-c) AFM images of MC, NC, and MNC, respectively. (d-f) Photographs

of water droplets sitting on MC, NC, and MNC, respectively. .................................. 29

Figure 2.3: Transmission spectra of ormosil coatings and an uncoated glass substrate.

All of the coatings are highly transparent at the visible wavelengths. The inset shows

a photograph of transparent coatings. ........................................................................ 30

Figure 2.4: Compression experiments using the NC as the sticky hydrophobic top

plate. Photographs of the water droplets compressed and released on micro-porous MC

(left column) and micro/nano-porous MNC (right column). ..................................... 32

Figure 2.5: (a) Contact-angle change of coatings during compression. b) Generated

Laplace pressures for coatings during compression. Wenzel transition was observed

for the MC at the first stages of compression; on the other hand, the Cassie state was

preserved in the MNC even at the highest compression ............................................ 33

Figure 2.6: Droplet squeezing between two identical surfaces. Contact angles of

surfaces before and after compression. The inset shows the squeezed water droplet

between two MNC surfaces. There is only a slight decrease in the contact angle of the

MNC after compression. The large decrease in the MC shows loss of the

superhydrophobic property. ....................................................................................... 34

Figure 2.7: Droplet squeezing between two identical surfaces. (a) Sliding angles of

surfaces before and after compression experiments for the MC (left column) and MNC

(right column). A droplet on the MC sticks to the surface after compression due to

Wenzel transition, whereas a water droplet on the MNC easily rolled off from the

surface at 15° tilting angle, indicating that the water droplet is still at the Cassie state.

(b) Contact-angle hysteresis of the surfaces before and after compression. The increase

in contact-angle hysteresis is significantly larger for the MC.................................... 35

Figure 2.8: Schematic representation of the proposed model for the excellent Cassie

state stability of the MNC coatings. For the MC, the contact line gradually slides down

from the walls of microporous structures with increasing pressure; after a critical

pressure difference, the contact line touches the hydrophilic glass substrate and pins to

xiii

the surface instantly (transition to the Wenzel state). On the other hand, for the MNC,

the bottom NC layer prevents interaction between the glass substrate and contact line.

.................................................................................................................................... 36

Figure 2.9: Evaporation experiments: Photographs of water droplets sitting on the MC

(upper row) and MNC (lower row) at different instants of the evaporation process.

Wenzel transition was observed for a droplet sitting on the MC at around 18 min. A

water droplet on the MNC, on the other hand, preserved its spherical shape even at the

final stages of evaporation, indicating the excellent Cassie stability of this coating. 37

Figure 2.10: Contact angles of the surfaces during evaporation experiment. The inset

shows the generated Laplace pressures at the point where contact angles of the surfaces

decrease below 120°. .................................................................................................. 38

Figure 2.11: Change of the water droplet contact-line diameter during evaporation. A

CCL mode was observed for the MC. For the MNC, sudden decreases in the contact-

line diameter were observed. This behavior, where the contact line of the droplet

retracts and the contact angle increases suddenly, is known as stick−slip (S−S)

behavior, referring to successive “pinning−depinning” of the droplet. ..................... 40

Figure 2.12: Force generation and relaxation of the contact line for the MNC. (a)

Depinning force generation and its relaxation during evaporation. Red areas indicate

the force generation, and blue areas indicate the relaxation regions. (b) Baseline

retraction for the MNC between 20 and 21 min. After baseline retraction, the baseline

diameter significantly decreased and the contact angle increased. ............................ 41

Figure 2.13:.Generation of force needed for depinning of contact line during

evaporation for microporous coating. No force relaxation was observed for this

coating. ....................................................................................................................... 42

Figure 3.1: (a) Scanning electron microscopy (SEM) image of the untreated ormosil

surface with the water droplet profile in the inset image. (b) SEM image of the UV/O-

treated ormosil surface for 60 min with completely spreading thin liquid film in the

inset image (WCA= water contact angle). ................................................................. 48

Figure 3.2: (a) X-ray photoelectron spectroscopy (XPS) survey spectra of the ormosil

surfaces before and after UV/O treatment for 60 min. (b) High-resolution C1s spectra

of the ormosil surfaces before and after UV/O treatment for 60 min. ....................... 49

xiv

Figure 3.3: Change in the water contact angle (WCA) values on treated ormosil

surfaces depending on the UV/O exposure time ranging from 5 to 60 min. Inset images

represent the droplet profiles recorded for 0, 10, 20, 30, 40, and 60 min from top to

bottom. Inset graph shows the linear fit for the decreasing behavior of WCA as a

function of treatment time. ......................................................................................... 50

Figure 3.4: Water contact angle (WCA) of (a) untreated and UV/O treated in (b) 15,

(c) 30 and (d) 60 min surfaces after 5 months respectively. ...................................... 51

Figure 3.5: (a) Schematic representation of UV/O treatment of superhydrophobic

ormosil surfaces (left) and chemical groups on the treated and untreated areas (right).

(b) Patterned ormosil coating on a glass substrate with completely spreading FITC

labeled BSA solution on the superhydrophilic area and a water droplet sitting on the

superhydrophobic area in a perfect spherical shape (dimesion of glass substrate is

2cm*3cm). Corresponding schemes indicate the complete wetting and non-wetting on

the treated and untreated areas, respectively. ............................................................. 52

Figure 3.6: (a) Ormosil surfaces on glass substrate with dyed water solutions in square

wetted patterns with 1 mm edge dimension. (b) Ormosil surfaces on glass substrate

with colored aqueous solutions in stripe patterns with 1 mm thickness. (Blue –

Methylene blue, Red- Rhodamine 6G, Green- Mixture of Acridine orange and

Methylene blue) ......................................................................................................... 53

Figure 3.7: High-throughput mixing of individual droplets. (a) Arrays of droplets dyed

with different colors (blue-Methylene blue and red-Rhodamine 6G) were formed by

individually dispensing dyed water to the wetted spots on two separate patterned

surfaces. Identical array sizes (a 4x6 array on glass substrates with the same size) were

used. Longer UV/O treatment (~60 min.) was applied to the bottom surface compared

to the top surface (~30 min.) to collect most of the mixed solution on the bottom

surface after mixing. (b) The patterned surfaces were aligned using a microstage.

Droplet arrays (c) during the contact and (d) after the contact. Each individual droplet

on the top surface mixed with its counterpart in the bottom surface. No lateral mixing

was observed between the droplets on the same surface demonstrating the precise

control of droplet behavior on the patterned surfaces. (e) Arrays of mixed droplets.

xv

Most of the mixed solution remained on the bottom surface (left) due to the gravity

and the higher wettability of the patterned regions. ................................................... 54

Figure 3.8: (a) High-density droplet arrays in circle patterns with 200 µm diameter. (b)

A bent cellulose acetate coated with ormosil and droplet arrays on the superhydrophilic

patterns of the ormosil. ............................................................................................... 55

Figure 3.9: Performance of 60 min-UV/O treated ormosil surface in wettability

contrast after storing for 5 months. Spherical droplet on the superhydrophobic region

and Rhodamine 6G aqueous solution on the superhydrophilic stripe patterns with

corresponding WCA values. ...................................................................................... 55

Figure 3.10: Fluorescent images taken from the BSA-adsorbed surfaces treated for (a)

15 min, (b) 30 min, and (c) 60 min using confocal microscopy. Samples were excited

at 488 nm and the emission was collected at 505 nm. Fluorescent signals denoted as

green in the images correspond to the emission of FITC in the wetted patterns whereas

the black background corresponds to the superhydrophobic regions with no fluorescent

signal. ......................................................................................................................... 57

Figure 3.11: (a) Normalized fluorescence intensity of the wetted patterns with respect

to UV/O treatment time. (b) WCA values on the UV/O treated ormosil surfaces with

respect to UV/O treatment time. ................................................................................ 58

Figure 3.12: (a) Fluorescent microscope image of GFP-expressing E. coli cells on the

superhydrophilic patterns of ormosil with square wetted patterns (1x1 mm) (b) Close-

up view of one wetted pattern with adhered bacteria. Fluorescent signal of GFP from

individual bacteria cells with ~2 µm dimensions were clearly observed................... 59

16

Chapter 1

Introduction

Biological surfaces such as the leaves of Lotus, wings of Morphobutterfly, legs of

water strider insects and the back of Namib Desert beetles have inspired the design of

artificial functional surfaces [1-3]. Functional surfaces are achieved by either

modifying the surface chemistry, texturing the surface morphology or both. In this

context, thin film coatings are particularly advantageous as they generally allow the

flexible and the precise control of surface properties relatively simple fabrication

methods. Accordingly, superhydrophobic coatings that try to mimick natural

structures gained considerable attention in the past decades. Variety of applications in

biology to industry; ranging from self-cleaning surfaces, [4, 5] oil/water, drag

reduction separation [2, 6] to water harvesting [7-10], microfluidics [11, 12] were

investigated. Over the past decade, we have witnessed many impressive reports

demonstrating design and fabrication of superhydrophobic surfaces using lithographic

methods, [13-15] sol-gel techniques, [4, 16-19] phase separation in polymer blends,

[20] electrospinning, [21, 22] and others [23-28]. There are considerable effort for

explaining the mechanisms behind the non-wetting behavior (i. e. Cassie state of

wetting) of such surfaces. However there are still some problems hindering the

practical applications of these surfaces. For example, currently available self-cleaning

surfaces generally fail in preserving the stability of non-wetting properties (i. e.

transition to sticky Wenzel state) over time, especially in outdoor applications, in

contrast with successful examples in natural surfaces [29-31]. Therefore, improving

the stability of Cassie state of wetting in self-cleaning coatings is one of the prerequisite

for utilizing self-cleaning surfaces in real life applications. Additionally, facile large

area fabrication methods that allow precise control over wettability are still needed for

17

the feasibility of these surfaces in real life applications [32-34]. Moreover, the precise

control over the wettability is critical for demonstrating the enhanced functionality of

these surfaces in applications such as selective biomolecular adsorption [35, 36] and

surface tension confined microfluidics [37-40].

In this thesis, we focused on the design and fabrication of organically modified silica

(Ormosil) based functional coatings for real life applications. We investigated the

increasing the non-wetting behavior of ormosil surfaces and demonstrated more stable

weting state superhydrophobic coatings. In addition, we demonstrated the simple

fabrication method of superhydrophilicly patterned ormosil surfaces with controlling

bioadhesion.

This thesis is organized in four chapters. Chapter 1 gives the motivation and

organization of thesis, and the important terms and topics are introduced. In Chapter

2, we reported a facile large area sol-gel method to obtain bioinspired micro/nano

structured transparent coatings with enhanced stability against external pressure. Three

different organically modified silica (ormosil) coatings; i) nano-porous hydrophobic

coating , ii) micro-porous superhydrophobic coating, and iii) double layer

superhydrophobic coating with nano-porous bottom and micro-porous top layers were

prepared on glass surfaces. We investigated stability of the Cassie state in coatings

with droplet compression and evaporation experiments, where external pressures as

high as a few thousands of Pa is generated at the interface. In Chapter 3, we reported

a facile fabrication method of superhydrophilicly patterned superhydrophobic ormosil

coatings. On the defined areas of the superhydrophobic ormosil coatings, wetted

micropatterns were produced using Ultraviolet/Ozone (UV/O) treatment which

modifies the surface chemistry from hydrophobic to hydrophilic without changing the

surface morphology. The degree of wettability of the coatings can be precisely

controlled depending on the UV/O exposure duration ranging entire wettability range.

Furthermore, the protein adhesion to the surface depending on the wettability was

studied. Finally, in chapter 4, general conclusions and outcomes are given.

18

1.1 Wetting Phenomenon

The interaction between a solid and a liquid phases is known surface wetting

behavior. Surface wetting is characteristic to the materials; chemistry of the material

define the surface has a tendency to wet or repel liquid. Contact angle is the angle

where the intersection between liquid-gas (LG) interface meets a solid-liquid (SL)

interface and it is used for characterization of surface wettability. In ideal smooth

surfaces, wetting is defined by Young’s equation. According to the equation; contact

surface tension of the solid-gas (γSG,), solid-liquid (γSL,) and liquid-gas (γLG,) in Figure

1.1 [41]. The materials are classified as hydrophilic (i.e., angle of the surface (θC) is

independent of the droplet size and directly related to the CA < 90°) and hydrophobic

(i.e., CA > 90°) in Young’s contact angle [42].

Figure 1.1: Schematic view of water contact angle and Young’s equation.

Although, almost all of real surfaces have some level of roughness; micro, nano or

molecule level; even they look macroscopic smooth. Roughness increases the contact

angle of hydrophobic surfaces, thus Young’s equation doesn’t work when roughness

increases. Wetting of the rough surfaces are described by two different wetting models;

Wenzel and Cassie-Baxter.[43, 44] In the Wenzel model, the surface is completely

wetted and water fills the rough space of the surface (Wenzel state), whereas in the

Cassie-Baxter model, the droplet wets the surface partially and air pockets form

between the surface and water droplet (Cassie State) in Figure 1.2 [1, 45-47]. The

Wenzel equation is written as:

cos θW = r cos θC 1.1

19

where θW is Wenzel’s CA on the rough surface, r is the roughness parameter: actual

area divided by projected area of the surface and θC is Young’s CA (contact angle of

smooth material of same surface) [43]. In the Cassie-Baxter model equation is written

as:

cos θCB = -1+ φ (1+cos θC) 1.2

where θCB is Cassie-Baxter’s CA, φ is fraction of wetted area to whole area and θC is

Young’s CA (contact angle of smooth material of same surface) [44].

Adhesion (water adhesion to the surface) of the Cassie state and Wenzel state is

very different from each other. In Cassie State, water droplet roll-off from surface due

to air trapping. On the other hand, in Wenzel state, water pinned to the surface. When

Cassie state irreversibly transform to the Wenzel state, it loses water repelling

behavior. Therefore, stable Cassie state is important for applications such as self-

cleaning window, anticorrosion coating and underwater drag reduction coatings.[5,

48]

Figure 1.2: Shematic of water droplet on rough surfaces. On the right Wenzel model and on

the left Cassie-Baxter model are shown. Adopted by Ref [1]

According the models, roughness is another factor to determine the wettability of

the surface besides surface chemistry. When increases the surface roughness of the

hydrophilic surface, the surface become more hydrophilic even superhydrophilic. On

the other hand, when increases roughness of hydrophobic surfaces, it is become more

hydrophobic or (superhydrophobic). The surfaces are classified 4 regimes according

to the apparent contact angle. Surfaces with WCA more than 90° and less than 150°

20

called hydrophobic surfaces (Figure 1.3 i). When water forms spherical droplet on the

surface and WCA is more than 150°, the surface termed superhydrophobic (Figure 1.3

ii). Surfaces with WCA more 10° to 90° are called hydrophilic (Figure 1.3 iii). When

water spreads and WCA is less 10°, the surface is called superhydrophilic (Figure 1.3

iv) [49].

Figure 1.3: Representation of 4 wettability regimes. (i) hydrophobic surface 90°<WCA<150°

(ii) superhydrophobic surface WCA>150° (iii) hydrophilic surface 10°<WCA<90° and (iv)

superhydrophilic surface WCA<10° respectively. Adopted by Ref [49]

1.2 Superhydrophilicly Patterned Superhydrophobic

Surfaces

In nature, the amazing examples of surfaces are found and inspires us to design new

functional surfaces. The Namib Desert beetles are one of them. The insect has bumpy

surface which contains hydrophobic waxy and hydrophilic non waxy regions; using

this ‘functional back’, they are able to collect water droplet from fog using this their

functional backs. Figure 1.4 shows adult female desert beetle, its hydrophilic non-

waxy region of the back and SEM of depressed area. Water droplets nucleates the

hydrophilic regions and then droplet grows to specific size after it detached and rolls

on the waxy regions to the beetle mouth.[50]

21

Figure 1.4: Desert beetle and its ‘functional back’. (a) Adult female desert beetle (b)

hydrophilic non-waxy region of the back and (c) Scanning electron micrograph of the textured

surface of the depressed areas. Adopted by Ref [50]

Desert beetle inspired functional surfaces (i,e.,extreme wettability contrast by

combination of superhydrophilic and superhydrophobic regions at the micro scale) are

used wide range of applications such as water harvesting [7-10], open-air microfluidics

[11-12] and surface tension-confined liquid flow in microfluidics.[37][38][39][40] In

particular, precise control of the wetted and non-wetted patterns on a single surface

allows the formation of ordered droplet arrays and easy droplet manipulation as well

as the immobilization and easy-handling of water-dispersed materials including

nanoparticles, biomolecules and cells [35-36]. Therefore, patterned superhydrophobic

surfaces have attracted significant research interest for high-throughput biological and

chemical screening [51-54]. Figure 1.5 shows example of the

superhydrophobic/superhydophilic micropatterned surfaces; surface-tension confined

microchannel, ordered droplet array and controlled bioadhesion [51-53].

Figure 1.5: Example of the applications of superhydrophobic/superhydophilic micropatterned

surfaces. (a) Surface tension confined microchannels: dye gradient. Adopted by Ref [51] (b)

close-positioned order droplet arrays. Adopted by Ref [52] (c) Fluorescent cells cultured on a

patterned substrate for 48 h. Adopted by Ref [53]

22

1.3 Organically Modified Silica Surfaces

Many successful methods are reported to make functional surfaces, among these

methods, organically modified silica are preferred due to superior properties such as

mechanical and thermal stability, flexibility and mild, cost-effective synthesis and

coating conditions.[17, 56] Organically modified silica (ormosil) is organic-inorganic

hybrid material that contains interconnected silaxone (Si-O-Si) backbone and organic

groups connected to the backbone which incorporate rigidity of inorganics and

flexibility of polymer, respectively [57]. It is produced from via the sol-gel reaction of

organotrialkoxysilanes monomers. Reaction mechanisms are shown in Figure1.6 The

monomer (empirical formula RSi(OR)3 ) first hydrolyzed via alkoxy (OR) group under

acid or base catalysis, then condensated under base or acid catalysis to forming gel

network and non-hydrolyzed organic functional (R) group remains same throughout

the reaction.[58, 59] Ormosils can possess the mechanical and thermal stability of

inorganic (silica) groups besides functionality and flexibility of organic groups.[60]

Figure 1.6: Sol-gel reaction mechanisms of ormosil. (a). Hydrolyzation of the organosilane

monomer. (b). Condensation hydrolyzed monomers

Ormosil properties such as wettability, porosity, flexibility are easily tailored with

using different type catalysis, solvent, monomers and reaction conditions accordingly

with the targeted application [4, 5, 17, 61, 62]. Example of functional surfaces, were

prepared from ormosil, are shown in Figure 1.7. Superhydrophobic and flexible

23

hydrophobic antireflective ormosil coatings were prepared on glass and polymer

substrates.[17, 61]

Figure 1.7: Functional surfaces prepared from ormosil. Photographs of (a) Superhydrophobic

thin film on glass substrate with water contact angle 178°. Adopted from Ref [17] (b) Bent

nanoporous film coated polymer substrate (PEI). (c) Nanoporous ormosil partially coated glass

slide. Coated parts reflect less light indicating anti-reflective property. Adopted from Ref [61]

24

Chapter 2

Transparent Superhydrophobic Coatings with

Robust Cassie State of Wetting

2.1 Introduction

The Cassie state of wetting provides a large apparent water contact angle with low

contact-angle hysteresis and results in a roll-off superhydrophobic surface, which is

desired for practical applications such as self-cleaning windows and solar panels [4,

23, 63], underwater drag reduction [48, 64], and anticorrosion coatings [16].

Although, the Cassie state of wetting (roll-off) can easily and irreversibly transform

to the Wenzel state of wetting (sticky) under external stimuli such as pressure,

vibration, droplet impact and droplet evaporation by complete filling of the air pockets

with water [29-31]. The poor stability of the Cassie state remains as an unsolved

drawback against practical applications of superhydrophobic surfaces [31].

Particularly, the applications that require outdoor or underwater operation, where

surfaces are exposed to external conditions such as droplet impact or hydrostatic

pressure, require a robust Cassie state. Several authors have investigated the

parameters that affect the stability of the Cassie state in superhydrophobic coatings

[23, 31, 65-78].

However, in almost all of these studies, lithographically fabricated silicon or

poly(dimethylsiloxane) (PDMS) pillars were used [31, 65-68, 70-74], which are not

suitable for practical applications because of their high cost, limited material choice,

25

and inefficiency for large-area fabrication. In addition, in none of these studies were

fabricated surfaces optically transparent, which is a prerequisite for many applications

of nonwetting coatings such as self-cleaning windows and solar cells in terms of the

appearance and device performance [4, 23]. Therefore, facile fabrication methods for

transparent superhydrophobic surfaces with a stable Cassie state of wetting are still

needed.

In this chapter, we investigated the effect of different levels of surface topography

on the stability of the Cassie state of wetting in large-area and transparent

superhydrophobic coatings. Three different organically modified silica (ormosil)

coatings, (i) nanoporous hydrophobic coating (NC), (ii) microporous

superhydrophobic coating (MC), and (iii) double-layer superhydrophobic coating with

nanoporous bottom and microporous top layers (MNC), were prepared on glass

surfaces. The stability of the Cassie state of coatings against the external pressure was

examined by applying compression/relaxation cycles to water droplets sitting on the

surfaces. The changes of the apparent contact angle, contact-angle hysteresis, and

sliding-angle values of the surfaces before and after compression cycles were studied

to determine the Cassie/ Wenzel transition behavior of the surfaces. In addition, water

droplets were allowed to evaporate from the surfaces under ambient conditions, and

the changes in the contact angles and contact-line diameters with increasing Laplace

pressure were analyzed. We demonstrated that, upon combination of coatings with

different levels of topography (i.e., MNC), it is possible to fabricate transparent

superhydrophobic surfaces with extremely stable Cassie states of wetting on glass

surfaces.

This work published on the ACS Applied Materials&Interfaces (2014, 6, 9680-

9688) journal. Reproduced with permission from The American Chemical Society.

2.2 Experimental Section

Materials: Methyltrimethoxysilane (MTMS), oxalic acid, and ammonium

hydroxide (25%) were purchased from Merck (Germany), and dimethyl sulfoxide

(DMSO) and methanol were purchased from Carlo-Erba (Italy). All chemicals were

used as received.

26

Preparation of NC: Ormosil coatings were prepared according to our previous

report [61]. Initially, 1 mL of MTMS was dissolved in 2 mL of DMSO, and 0.5 mL of

an oxalic acid solution (10 mM) was slowly added to the mixture and stirred for 30

min. Then 0.42 mL of an ammonia solution (25%) and 0.19 mL of water in 5 mL of

DMSO were added, and the solution was further stirred for 15 min. Finally, the

solution was left for gelation at 25 °C. The gels are typically formed in about 1 h. After

gelation, approximately 20 mL of methanol was added onto the gels and incubated for

at least 6 h at 25 °C to remove DMSO and unreacted chemicals. This treatment was

repeated for four times to ensure complete removal of DMSO and other chemical

residues. After washing, 12 mL of methanol was added onto the gels and gels were

sonicated using an ultrasonic homogenizer for 45 s at 20 W to obtain ormosil colloids

that are suitable for thin-film deposition. Then ormosil colloids were spin-coated on

clean glass substrates at 2000 rpm and dried at room temperature overnight. To

increase the integrity and hydrophobicity, coatings were heat-treated at 450 °C for 1

h.

Preparation of MC: The superhydrophobic microporous coatings were prepared

according to our previous report [17]. First, 1 mL of MTMS was dissolved in 9.74 mL

of methanol. Then 0.5 mL of an oxalic acid solution (1 mM) was added to the mixture

and gently stirred for 30 min. The mixture was left for hydrolysis for 24 h at room

temperature. After hydrolysis, 0.42 mL of an ammonia solution (25%) and 0.19 mL of

water were added slowly to the mixture, and the reaction mixture was stirred for 15

min. The solution was left for 2 days at 25 °C for gelation and aging. After aging of

the gel network, 12 mL of methanol was added onto the gels and gels were sonicated

using an ultrasonic homogenizer for 45 s at 20 W to obtain ormosil colloids. Then

ormosil colloids were spin-coated on clean glass substrates at 2000 rpm and dried at

room temperature overnight. To increase the integrity and hydrophobicity, coatings

were heat-treated at 450 °C for 1 h.

Preparation of MNC: A double-layer coating was prepared by successive spin

coating of ormosil colloids. Initially, a nanoporous layer (bottom layer) was spin-

coated on clean glass substrates at 2000 rpm and dried at room temperature overnight.

Then a microporous layer (top layer) was spin-coated on the nanoporous layer at 2000

27

rpm and dried at room temperature overnight. Finally, double-layer coatings were

cured at 450 °C for 1 h.

Characterization: The surface topography of the coatings was investigated with

scanning electron microscopy (E-SEM; Quanta 200F, FEI) at high vacuum after

coating with 5 nm of gold or platinum. Atomic force microscopy (AFM; XE-100E,

psia) was used in noncontact mode to characterize the surface morphology and

roughness of the coatings. Root-mean-square (rms) roughness values were calculated

from three separate AFM images of the coatings, all of which were obtained from 10

× 10 μm2 area. The thicknesses of the coatings were measured with an ellipsometer

(V-Vase, J. A. Woollam). Optical transmission experiments were performed with a

UV−vis spectrophotometer (Cary 5000, Varian). Contact-angle measurements were

performed using a contact-angle meter (OCA 30, Dataphysics). For static contact-

angle measurements, 10 μL of water droplets was used with the Laplace−Young

fitting. Advancing and receding angles were measured by the addition and subtraction

of 5 μL of water to/ from the 10 μL of droplets sitting on the surfaces. Sliding-angle

measurements were performed using 10 μL of water droplets.

2.3 Preparation and Characterization of Ormosil

Coatings

MC and NC transparent ormosil coatings on glass surfaces were prepared according

to previous reports [17, 61]. MNCs were prepared by the subsequent coating of these

two ormosil films: a dual-layer coating with NC as the bottom layer and MC as the top

layer.

The Figure 2.1 shows the SEM images of ormosil surfaces. MC demonstrated

microporous and rough surface topography (Figure 2.1 a, d). An underlying glass

substrate can be observed through the openings of the coating (orange outlined area in

Figure 2.1 a). On the other hand, NC fully covers the surface and demonstrates

nanoporous and uniform surface topography (Figure 2.1 b, e). Both layers of MNC

coating can be clearly distinguished in the SEM image: rough microporous structure

of MC at the top and nanoporous NC at the bottom (Figure 2.1 c, f). Lower-

28

magnification SEM images are provided in the Figure 2.1 d-f to demonstrate the

uniformity of the coatings.

Figure 2.1: SEM images of ormosil coatings. (a-c) High magnification SEM images of MC,

NC, and MNC, respectively. The glass surface exposed through the openings in MC is outlined

in orange. In the MNC, the bottom NC is outlined in orange. Low magnification SEM images

of the coatings. (d-f) MC, NC and MNC. Arrows indicate the underlying glass substrate in (d)

and NC in (f).

Average surface roughness values of the coatings were calculated from their three

separated AFM images (10 × 10 μm2) and are given in the first row of Figure 2.2 a-c

with the representative AFM images of the surfaces. As can be expected, MC and NC

have high (218 ± 18 nm) and low (11 ± 2 nm) rms surface roughness values,

respectively. The surface roughness of double-layer MNC is on the same order with

MC, 248 ± 3 nm.

Because of the low-energy surface methyl groups, coatings are intrinsically

hydrophobic. In addition to the hydrophobic surface chemistry, their

micro/nanostructured surfaces provide superhydrophobicity. The MC demonstrates

roll-off superhydrophobicity (Cassie state) because of its rough and hydrophobic

29

surface with a water contact angle of 162° and a sliding angle of smaller than 1° (Figure

2.2 d). In comparison, even if the NC surface is also highly hydrophobic with a water

contact angle of 145°, water droplets pinned to the surface (Wenzel state) because of

its low surface roughness (Figure 2.2 e). MNC demonstrated a Cassie state

superhydrophobic property with water contact and sliding angles similar to those of

the MC (Figure 2.2 f).

Figure 2.2: (a-c) AFM images of MC, NC, and MNC, respectively. (d-f) Photographs of water

droplets sitting on MC, NC, and MNC, respectively.

Transparency of superhydrophobic coatings is desired for many practical

applications including self-cleaning windows and solar panels. [4, 23, 25, 63, 79-81]

Figure 2.3 shows the transmission spectra of the ormosil coatings. Despite its high

surface roughness value, MC demonstrates high transparency (up to 85%) in the

visible wavelengths. The slight decrease in the transmission compared to the bare glass

is due to the light scattering from its microporous surface, which is more pronounced

in smaller wavelengths.[82] The transmission of NC is even higher than bare glass

(i.e., antireflection property) because of its smooth surface and porous nature, which

introduces a gradual refractive index change and reduces the reflectance of the glass.[4,

30

61, 83] Lastly, the transmission spectrum of double-layer MNC is almost identical

with that of MC because of the same order of their surface porosity and roughness.

Figure 2.3: Transmission spectra of ormosil coatings and an uncoated glass substrate. All of

the coatings are highly transparent at the visible wavelengths. The inset shows a photograph

of transparent coatings.

2.4 Compression Experiments

Squeezing water droplets between two surfaces is often used to investigate the

stability of the Cassie state of wetting of superhydrophobic coatings against the

external pressure.[46, 69, 70, 76] During compression of a droplet placed on a

superhydrophobic surface, pressure differences as high as 300 Pa can be generated

depending on the distance between two surfaces and the apparent contact angles of the

surfaces.[46] The pressure difference generated during compression can be calculated

using the following Laplace equation, when droplet radius is much higher than

interplanar distance:[46]

31

∆P = γ(cosθb + cosθt)/x, for x<< R, 2.1

where γ is the surface tension of water, R is the radius of the droplet, x is the gap

between the top and bottom surfaces, and θb and θt are the water contact angles of the

bottom and top surfaces, respectively.

For compression experiments, first a droplet (10 μL) was placed on a NC surface

(top plane) and slowly approached a bottom plate (MC or MNC) to squeeze the water

droplet between two surfaces up to an interplanar distance (Δx) of ∼0.4 mm. Then we

slowly relaxed the top surface. Figure 2.4 shows the frames of the water droplets

during squeezing and relaxation between NC and MC (left column) and NC and MNC

(right column). In the first stages of compression (the second row in Figure 2.4), the

bottom contact lines largely preserved their high water contact angle values for both

MC and MNC. However, for the highest compression (the third row in Figure 2.4), a

significant decrease in the apparent contact angle of MC was observed, which suggests

a transition between the Cassie and Wenzel states. [46, 69] On the other hand, double-

layer MNC preserved the high water contact-angle value even at this high

compression. The wetting difference between two coatings is more obvious during

relaxation. It can be clearly seen from the fourth and fifth row in Figure 2.4 that after

compression a water droplet strongly sticks to the MC surface, indicating the Wenzel

transition; quite the reverse, a droplet on the MNC surface relaxes easily without

changing its spherical shape, indicating that the MNC surface preserved the Cassie

state.[69, 70] Figure 2.5a shows the in situ contact-angle measurements during

compression of the surfaces, and Figure 2.5b shows the generated pressure as a

function of Δx. The contact angle of the MC sharply decreased below 150° in the first

stages of squeezing. On the other hand, the water contact angle of the MNC is larger

than 150° even at the lowest Δx value. The generated Laplace pressure difference (eq

2.1) at highest compression when interplanar distance x=0.4 mm (Figure 2.5b) is

almost 2-fold higher for the MNC because it demonstrates higher water contact-angle

values than the MC during compression. From Figure 2.5b, it can be observed that, for

the MC, Cassie to Wenzel transition occurred at a pressure difference of roughly

around 80 Pa, whereas no transition was observed for the MNC even at a pressure

difference of 260 Pa.

32

Figure 2.4: Compression experiments using the NC as the sticky hydrophobic top plate.

Photographs of the water droplets compressed and released on micro-porous MC (left column)

and micro/nano-porous MNC (right column).

33

Figure 2.5: (a) Contact-angle change of coatings during compression. b) Generated Laplace

pressures for coatings during compression. Wenzel transition was observed for the MC at the

first stages of compression; on the other hand, the Cassie state was preserved in the MNC even

at the highest compression

Besides the decrease in the apparent contact angle, increases in the contact-angle

hysteresis and sliding angle are indicators of Cassie to Wenzel transition. [47] Surfaces

with the Cassie state of wetting demonstrate low contact-angle hysteresis values and

smaller sliding-angle values due to easy roll-off of the droplets; however, for the

Wenzel state of wetting, droplets pin to the surfaces with high contact-angle hysteresis

34

values. To measure the water contact and sliding angles and contact-angle hysteresis

of MC and MNC after droplet relaxation, we designed a compression experiment

where a water droplet (10 μL) is squeezed between two identical surfaces (MC or

MNC; Figure 4, inset) and released. Figure 2.6 shows the contact angles of surfaces

after release from a compression distance of 1 mm. The contact angle of a released

droplet on the MC sharply decreases to 127°, which is in good accordance with initial

compression experiments. In contrast, there is only 5° difference in the contact angles

of the MNC before and after squeezing. Figure 2.7a shows the water droplet sliding

angles before and after the compression test. Before squeezing, both surfaces were

highly water repellent with sliding angles lower than 1°. However, a droplet on the

MC strongly sticks to the surface after pressure release, and it remains pinned even at

90° tilting. For the MNC, although an increase in the sliding angle after pressure

release was observed, the droplet can easily roll off from the surface at 15° of tilting.

Figure 2.7b shows the contact-angle hysteresis of the surfaces before and after

compression. Both surfaces have contact-angle hysteresis values of around 2° before

compression. After compression, the increase in the contact-angle hysteresis is much

more pronounced for MC (∼13°) than for MNC (∼7°).

Figure 2.6: Droplet squeezing between two identical surfaces. Contact angles of surfaces

before and after compression. The inset shows the squeezed water droplet between two MNC

surfaces. There is only a slight decrease in the contact angle of the MNC after compression.

The large decrease in the MC shows loss of the superhydrophobic property.

35

Figure 2.7: Droplet squeezing between two identical surfaces. (a) Sliding angles of surfaces

before and after compression experiments for the MC (left column) and MNC (right column).

A droplet on the MC sticks to the surface after compression due to Wenzel transition, whereas

a water droplet on the MNC easily rolled off from the surface at 15° tilting angle, indicating

that the water droplet is still at the Cassie state. (b) Contact-angle hysteresis of the surfaces

before and after compression. The increase in contact-angle hysteresis is significantly larger

for the MC.

The excellent Cassie state stability of the MNC can be attributed to its double-layer

structure (Figure 2.8). For the MC, the contact line gradually slides down from the

walls of microporous structures with increasing pressure; after a critical pressure

difference, the contact line touches the hydrophilic glass substrate and pins to the

surface instantly (transition to the Wenzel state). On the other hand, for the MNC the

bottom NC layer prevents interaction between the glass substrate and contact line.

Under a critical pressure difference, the contact line must touch the NC surface, and at

36

that point, transition to the nano-Cassie state from the initial micro-Cassie state can be

expected.[74] After removing the pressure, contact line can partially recover to the

initial micro-Cassie state. The increase in contact-angle hysteresis and the sliding angle

and the slight decrease in the apparent contact angle support the proposed partial

transition model between nano- and micro-Cassie states.

Figure 2.8: Schematic representation of the proposed model for the excellent Cassie state

stability of the MNC coatings. For the MC, the contact line gradually slides down from the

walls of microporous structures with increasing pressure; after a critical pressure difference,

the contact line touches the hydrophilic glass substrate and pins to the surface instantly

(transition to the Wenzel state). On the other hand, for the MNC, the bottom NC layer prevents

interaction between the glass substrate and contact line.

2.5 Evaporation Experiments

During evaporation of water droplets from superhydrophobic surfaces, Laplace

pressures as high as a few thousand pascal can be generated because of size reduction

of the droplet, which is around 1 order of magnitude larger than the pressure difference

37

generated during compression experiments.[84, 85] Therefore, to further demonstrate

the Cassie state stability of the MNC, we performed evaporation experiments by

placing water droplets (6 μL) to the surfaces and allowing their evaporation at ambient

conditions (approximately at 22 °C and 23% humidity).

Figure 2.9 shows time-dependent evaluation of the shape of water droplets on the

MC and MNC. A water droplet on the MC lost its spherical shape gradually during

evaporation and pinned to the surface, which indicates the Wenzel transition. On the

other hand, a water droplet on the MNC preserved its spherical shape even at the last

stages of evaporation, which shows that the water droplet was still at the Cassie state.

Figure 2.9: Evaporation experiments: Photographs of water droplets sitting on the MC (upper

row) and MNC (lower row) at different instants of the evaporation process. Wenzel transition

was observed for a droplet sitting on the MC at around 18 min. A water droplet on the MNC,

on the other hand, preserved its spherical shape even at the final stages of evaporation,

indicating the excellent Cassie stability of this coating.

Figure 2.10 shows the apparent contact-angle change of the surfaces during

evaporation. The contact angle of the water droplet on the MC continuously decreased

to 120° in about 22 min and sharply decreased to almost 0° after approximately 30

min, where the droplet completely evaporated. The contact angle of the water droplet

on the MNC decreased similarly to the MC for the first 10 min. However, at this point,

a jump in the contact angle occurred and it almost completely recovered to its initial

value. Then it started to decrease again, and a second jump occurred after 19 min.

Similarly, a third jump was observed after 31 min. At the 35th minute, the size of the

38

droplet largely reduced, and after approximately 10 s, it completely evaporated. The

reason for this behavior will be discussed in more detail below. Note that the time

required for complete evaporation of a droplet is slightly lower on the MC compared

to the MNC, which is due to the increased evaporation rate after Wenzel transition

(i.e., improved wettability) for this coating. To compare the Cassie state stability of

coatings during droplet evaporation, we calculated the Laplace pressures [65] (ΔP =

2γ/R, where γ is the surface tension of water and R is the droplet radius with a spherical

shape) at the points where the contact angle of the water droplets decreases to around

120° (t = 21 and 35 min for the MC and MNC, respectively; Figure 6b, inset). The

Laplace pressure at this point for the MNC (1620 Pa) is almost 7-fold higher than that

of the MC (248 Pa). We note that the Cassie state stability of the MNC is even better

than that of the Lotus leaf, where the same transition was observed slightly above 400

Pa [65]. The difference between the pressure values where the contact angles of the

MC and MNC surfaces and Lotus leaf reached 120° demonstrates the enhanced Cassie

state stability of the MNC surface. The contact-angle decrease rate of the MNC surface

is much slower compared to that of both the MC surface and Lotus leaf, and Wenzel

transition occurs at much later stages of evaporation.

Figure 2.10: Contact angles of the surfaces during evaporation experiment. The inset shows

the generated Laplace pressures at the point where contact angles of the surfaces decrease

below 120°.

39

For droplet evaporation experiments, four evaporation behaviors were defined in

the previous studies:[67, 86-92] (i) the constant-contact-line (CCL) state, where the

contact-line diameter is constant and the contact angle is decreasing, (ii) the constant-

contact-angle (CCA) state, where the contact angle is constant and the contact-line

diameter is decreasing, (iii) the stick−slip (S−S) behavior, where the droplet baseline

retracts step by step, resulting in sudden increases in the contact angle,[92] and (iv),

the mixed state, where both the contact angle and contact-line diameter are decreasing.

To identify the evaporation modes of our surfaces, we investigated the contact-line

diameters of droplets during evaporation (Figure 2.11). For the MC, the CCL mode

was observed in the first 25 min, where the diameter of the contact line is almost

constant and the apparent contact angle is constantly decreasing (Figure 2.10 and

2.11). Also, at the last stages of evaporation, a mixed mode was observed. For the

MNC, more complicated evaporation modes were observed (Figure 2.11). In the first

10 min, a CCL mode similar to that for the MC was observed. However, after this

point, a sudden decrease in the diameter of the contact line was observed. Note that a

sudden decrease in the contact-line diameter occurred at the same time with a jump in

the contact angle. This behavior has been observed previously on rough hydrophobic

surfaces and is called as “stick−slip”, referring to successive “pinning−depinning” of

droplets during evaporation.[71, 93-95] As in contact-angle measurements, there are

three abrupt changes in the baseline diameter of the MNC where the S−S behavior was

observed, and between these jumps, the contact line can be approximated to be

constant (CCL mode). Also, at the last stages of evaporation, the mixed mode was

observed.

40

Figure 2.11: Change of the water droplet contact-line diameter during evaporation. A CCL

mode was observed for the MC. For the MNC, sudden decreases in the contact-line diameter

were observed. This behavior, where the contact line of the droplet retracts and the contact

angle increases suddenly, is known as stick−slip (S−S) behavior, referring to successive

“pinning−depinning” of the droplet.

The CCL modes observed for both coatings can be explained by analyzing the

forces at the triple contact line.[67, 96] At t = 0, a droplet is in the equilibrium and the

contact line is fixed. Then water starts to evaporate and therefore the droplet volume

is continuously decreasing, which results in a decrease in the contact angle. The

decrease in the contact angle disturbs the equilibrium between forces acting on the

droplet and generates a radial force toward the center of the drop. This force (i.e.,

depinning force) retracts the contact line and recovers the initial contact angle. The

depinning force (FD) can be expressed as [96]:

FD = 2Rbσlg (cosθt - cosθi) 2.2

where Rb is contact-line radius, σlg is the surface tension of water, θt is the contact angle

at a certain interval during evaporation, and θi is the initial contact angle. Figure 2.12a

shows the depinning force generated during the evaporation process for the MNC. We

observed that, during evaporation, first a depinning force around 5 mN was generated

41

and then the droplet relaxed by retraction of the contact line, and this cycle was

repeated three times. Figure 2.12b shows the contact-line retraction between the 20th

and 21th minutes. After contactline retraction, the apparent contact angle of the droplet

reaches its initial value, which indicates total relaxation. Note that no relaxation was

observed for the MC even at depinning forces larger than 100 mN, which shows that

the water droplet on the MC strongly pins to the surface during evaporation (see the

Figure 2.13).

Figure 2.12: Force generation and relaxation of the contact line for the MNC. (a) Depinning

force generation and its relaxation during evaporation. Red areas indicate the force generation,

42

and blue areas indicate the relaxation regions. (b) Baseline retraction for the MNC between 20

and 21 min. After baseline retraction, the baseline diameter significantly decreased and the

contact angle increased.

Figure 2.13:.Generation of force needed for depinning of contact line during evaporation for

microporous coating. No force relaxation was observed for this coating.

43

Chapter 3

Patterning Superhydrophobic Ormosil

Surfaces for Controlled Wetting and

Bioadhesion

3.1 Introduction

Over the past decade, a number of successful methods have been introduced to

prepare functional superhydrophilic-superhydrophobic micropatterned surfaces

including photo-induced chemical modification, [53, 97-100] plasma treatment,[8,

101-103] ink-printing,[10, 104-106] direct laser writing,[56] UV exposure [107-109]

and photo-catalytic decomposition.[9, 110-112] However, most of the above-

mentioned methods use at least two or more production steps; e.g. pre-modification of

the surfaces with either hydrophilic or hydrophobic materials, followed by post-

modification to form wetted and non-wetted patterns, which complicates the

fabrication. In addition, stability and control over the degree of wettability are

important for practical applications of the patterned superhydrophobic surfaces.

Particularly for biological systems, wettability plays a key role on the control of cell

adhesion and protein adsorption.[32-34]

In an alternative approach, Mano and co-workers prepared patterned

superhydrophobic polymeric platforms with well-controlled wettability using UV/O

treatment which offers simplicity and reduced cost.[11, 54] However, when UV/O is

44

applied on polymer surfaces, contact angle increases over time (hydrophobic recovery

effect) due to the flexibility and reorganization of the bonds within the polymer chains

[113-115].

In this chapter, the facile generation of robust hydrophilic patterns on

superhydrophobic organically modified silica (ormosil) surfaces using a one-step rapid

method was introduced. Superhydrophobic hybrid silica surfaces have the potential to

address the above-mentioned problems; they eliminate the modification steps prior to

patterning since hydrophobic surface chemistry and high surface roughness were

combined in one-pot approach. They also exhibit higher chemical stability as well as

easier functionalization relative to polymers. In order to create fully isolated wetted

and non-wetted patterns, certain regions of the as-prepared surfaces were treated with

UV/O which modified the surface two-dimensionally by switching its chemistry from

methyl groups to hydrophilic silanol groups without changing morphology. We

demonstrated that the degree of wettability of the hydrophilic patterns can be

controlled depending on the UV/O treatment duration. Extremely hydrophilic regions

with water contact angles (WCA) nearly 0º can be formed while the unmodified

surface domains exhibit WCA close to 170º. One can rapidly form arrays of droplets

on the patterned surfaces by dipping them into aqueous media or can dispense

microdroplets individually to distinct micron-sized superhydrophilic spots. We further

demonstrate that biomolecules such as albumin can selectively adsorb and be confined

on the hydrophilic domains where the degree of wettability significantly influences the

amount of adsorption. Superhydrophilic-superhydrophobic patterns are also intact

under bending impacts owing to the robustness and flexibility of ormosil coatings. In

addition, patterned ormosil coatings retained their wettability performance for the

storage periods of several months. Superhydrophilic-patterned ormosil coatings can be

potentially used in drug screening, cell microarrays and microfluidics with their low-

cost preparation, wide pattern diversity and robustness.

45

3.2 Experimental Section

Materials. Methyltrimethoxysilane (MTMS), oxalic acid and ammonium

hydroxide (25%) were purchased from Merck (Germany), methanol was purchased

from Carlo-Erba (Italy), fluorescein isothiocyanate-bovine serum albumin conjugate

(FITC-BSA, with MW of 66 and 389.4 kDa for BSA and FITC, respectively) was

purchased from Sigma-Aldrich (USA). All chemicals were used as received. Ultra-

pure water (18.2 MΩ.cm at 25°C) obtained using a Millipore Milli-Q water

purification system (Billerica, USA) was used in all experiments.

Preparation of Superhydrophobic Ormosil Coatings. Ormosil superhydrophobic

coatings were prepared via two step sol-gel reaction as introduced in our previous

report.[17] First, 1 mL of MTMS was dissolved in 9.74 mL of methanol. Following

15 min stirring of the solution, 0.5 mL of 1 mM oxalic acid solution was added

dropwise and the solution gently stirred for 30 min. Then, the mixture was left to

hydrolyze the organosilane precursor completely for 24 h at room temperature. After

the hydrolysis step, 0.19 mL of water and 0.42 mL of ammonia solution (25 %) were

added slowly as the catalyst of the reaction and the mixture was stirred for further 15

min. The resultant mixture was left for 2 days at 25 °C for complete gelation and aging.

After aging the gel network, 9 mL of methanol was added onto the gel and gel was

sonicated using an ultrasonic liquid processor for 45 s at 20 W to obtain

homogeneously dispersed ormosil colloids. The ormosil colloids were coated on

various clean substrates (glass, silicon, polymer) at 3000 rpm for 45 s using spin-coater

and were dried at room temperature overnight.

Mask preparation. Commercial cellulose acetate papers were patterned with CO2

laser cutter (Zing laser system, Epilog laser, U.S) to obtain hollow masks.

Preparation of superhydrophilic and patterned surfaces. Superhydrophobic

ormosil coated surfaces were exposed UV-ozone for certain time (5-60 min) using

commercial surface cleaning system (PSD-UV, Novascan Technologies, U.S) at

ambient condition without extra oxygen. The cleaning system was used mercury lamp

with wavelength 254 nm and 185nm. Superhydrophobic surface was covered with

hollow mask and treated with UVO for specific time (15, 30, 60 min).

46

Protein Adsorption Studies. Patterned surfaces with 1 mm diameter circular spots

were prepared UVO with different exposure time and dipped to the 1mg/ml FICT-

BSA solution for 30s and then gently washed with ultra-pure water.

Bacterial Adhesion. GFP-(green fluorescent protein) expressing E. coli with size of

2 μm was proliferated in LB media at 37 °C under CO2 environment. Then, LB was

precipitated and bacteria was dispersed in PBS (phosphate buffer solution, pH~7.4)

medium. Approximately 200 μL of the bacteria suspension was rolled from the

superhydrophilic-patterned ormosil surface and tiny droplets of the suspension was

distributed to the wetted spots.

Characterizations. XPS, SEM, CA, Confocal, Image J Chemical analysis of the

surfaces were performed using X-ray photoelectron spectroscopy(XPS) (Thermo

Fisher Scientific, U.K) with Al K-α monochromator (0.1 eV step size, 12kV, 2.5mA,

spot size 400 µm) and an electron take-off angle of 90°. Avantage software package

was used peak calibration and peak fitting. Surface morphology of the surfaces were

analyzed with Scanning Electron Microscope (SEM) (E-SEM, Quanta 200F, FEI)

under high vacuum condition at 10kV after coated with 6 nm thick gold-platinium

layer. Static contact angle (WCA) of the surfaces were measured using contact angle

meter (OCA 30, Dataphysics). 4 µL water droplets were used for the measurements

and the measurements were analyzed with Laplace-Young. Confocal images were at

10X objectives taken with a confocal microscope (Model LSM 510, Zeiss, Germany).

FITC-BSA was excited with Argon laser at 488 nm and emission collected over 505

nm with photomultiplier tubes. ImageJ program was used quantifying fluorescent

intensity.

3.3 UV/Ozone Treated and Untreated Surfaces:

Preparation and Characterization

Superhydrophobic coatings were prepared from organically modified silica

(ormosil) hybrid nanostructures and then patterned as superhydrophilic-

superhydrophobic using a rapid and simple UV/O treatment step where hydrophobic

organic moieties were gradually decomposed. Ormosil nanostructures were

47

synthesized via a template-free sol-gel reaction: acid-catalyzed hydrolysis followed by

base-catalyzed condensation using methyltrimethoxysilane (MTMS) as the

organosilane precursor.[17] Methoxy groups of the organosilane monomers are

hydrolyzed and condensed through siloxane bonds forming silica network.[59] On the

other hand, the organic methyl groups remain non-hydrolyzed and unmodified

throughout the whole sol-gel process. Continued crosslinking of the networks leads to

the formation of ormosil gel which is a highly porous structure due to the macroscopic

phase separation of the hybrid particles.[57, 60, 61] After complete gelation, the gel

was broken down to form colloidal ormosil solution which was then coated onto glass

substrates. The resulting coatings preserved the initial porosity of the gel since methyl

groups in the interconnected network prevents the pore collapse during solvent

evaporation.[116, 117]

The surface morphologies of ormosil thin films were investigated by SEM. In

Figure 3.1a, the SEM of the as-prepared coating revealed the highly porous and phase-

separated ormosil structure with both nanometer and micrometer pores. The rough and

porous structure combined with the low surface-energy methyl group led to

superhydrophobicity of the coating with a static water contact angle (WCA) of 170°

(see the inset in Figure 3.1a). When the superhydrophobic surfaces were treated under

UV/O, the contact angle started to decrease, and after 1 h treatment, superhydrophilic

surfaces with water contact angles of nearly 0° were obtained (see the inset in Figure

3.1b). Figure 1b shows the SEM image of a coating after UV/O treatment for 1 h. The

surface morphology and porous structure remained unchanged as the initial state upon

treatment (Figure 3.1b).

48

Figure 3.1: (a) Scanning electron microscopy (SEM) image of the untreated ormosil surface

with the water droplet profile in the inset image. (b) SEM image of the UV/O-treated ormosil

surface for 60 min with completely spreading thin liquid film in the inset image (WCA= water

contact angle).

To investigate the underlying chemical mechanism behind the hydrophilic

modification in detail, we analyzed untreated and UV/O treated surfaces using X-Ray

photoelectron spectroscopy (XPS). Survey analysis showed that untreated ormosil

surface structure was composed of approximately 37.3% oxygen, 20.6% carbon, and

42.1% silicon revealing the organic-inorganic hybrid structure of ormosil. It is well

known that in the presence of oxygen, UV light (185 and 254 nm) generates ozone and

reactive atomic oxygen atoms which cleave surface-bound hydrocarbons forming

hydrophilic hydroxyl (-OH) groups [118, 119]. Accordingly, chemical composition of

the surface that was treated with UV/O for 1 h, was measured to be 51.9% oxygen,

6.3% carbon, and 41.8% silicon, respectively which indicated the decomposition of

surface methyl groups and oxygen enrichment on the surface (Figure 3.2a). Figure

3.2b shows the high resolution C1s spectra of the untreated and 60-min UV/O treated

ormosil surfaces. Peaks at 284.5 and 285.8 eV correspond to the C-H bond of methyl

groups and C-O bonds of non-hydrolyzed methoxy (-O-CH3) groups, respectively. The

decrease of both C-H and C-O bond intensities upon treatment revealed that besides

methyl groups, significant amount of the non-hydrolyzed methoxy groups were also

decomposed.

49

Figure 3.2: (a) X-ray photoelectron spectroscopy (XPS) survey spectra of the ormosil surfaces

before and after UV/O treatment for 60 min. (b) High-resolution C1s spectra of the ormosil

surfaces before and after UV/O treatment for 60 min.

Control of the degree of wettability, i.e. precise tuning the water contact angle of

the surfaces in a wide range from superhydrophobic to superhydrophilic is very

important for various applications such as liquid transport, droplet handling, and

biomolecular adsorption.[54] Previously, surface wettability was tuned on template-

synthesized nanoporous silica films from hydrophobic to hydrophilic with WCA range

from 100° to 10° and on polymer surfaces from superhydrophobic to superhydrophilic

with WCA range from 155° to 2° as a function of UV/O treatment time.[11, 118]

Herein, we demonstrate the good control of the wettability of the ormosil films from

superhydrophobic to extremely hydrophilic (with WCA range from 170° to 0°) by

adjusting UV/O treatment duration. WCA of the superhydrophobic ormosil coating

decreased with increasing UV/O exposure time and superhydrophilicity (WCA~0°)

was achieved after ~45 min (Figure 3.3). The profile of water droplet changed from

nearly spherical to semi-spherical within 20 min on the treated surface, and then to a

very thin liquid film after 45-60 min (Figure 3.3).

50

Figure 3.3: Change in the water contact angle (WCA) values on treated ormosil surfaces

depending on the UV/O exposure time ranging from 5 to 60 min. Inset images represent the

droplet profiles recorded for 0, 10, 20, 30, 40, and 60 min from top to bottom. Inset graph

shows the linear fit for the decreasing behavior of WCA as a function of treatment time.

Unlike the polymer surfaces which return to their hydrophobic state over time [114,

115] treated ormosil surfaces exhibited high stability in their wettability confirmed

with no significant change in WCA after 5 months. In figure 3.4, WCA of ormosil

surfaces untreated and treated UV/O with 15, 30 and 60 min after 5 months are shown.

51

Figure 3.4: Water contact angle (WCA) of (a) untreated and UV/O treated in (b) 15, (c) 30 and

(d) 60 min surfaces after 5 months respectively.

3.4 Superhydrophilicly Patterned Ormosil Surfaces

We utilized the simplicity of UV/O illumination for creating wetted patterns on the

superhydrophobic ormosil coatings. Process for the generation of patterned surfaces is

schematically shown in Figure 3.5a. As-prepared superhydrophobic coatings were

covered with a thin cellulose acetate (CA) mask with laser-cut hollow areas and treated

under UV/O. Mask are prepared as same size with the substrate (substrate dimension

2cm*3cm). Although superhydrophilicity can be obtained within 45 min without using

mask, the average treatment duration was chosen as 60 min during patterning in order

to ensure complete treatment at the pattern interfaces. Regions under the hollow sites

of the mask became exposed to UV/O whereas remaining parts were well shielded by

the CA mask. Replacement of methyl groups with hydroxyl groups in UV/O-exposed

areas increased the degree of wettability. On the other hand, chemical groups in the

untreated regions remained as original leading to the full separation of the

superhydrophobic and wetted domains (Figure 3.5a). The spherical water droplet

sitting on the untreated region indicated the conservation of the superhydrophobic

52

property whereas aqueous solution (~1% FITC-BSA) spreading on the treated area

with exactly the same shape of the hollowed mask confirmed the complete wetting

behavior (Figure 3.5b).

Figure 3.5: (a) Schematic representation of UV/O treatment of superhydrophobic ormosil

surfaces (left) and chemical groups on the treated and untreated areas (right). (b) Patterned

ormosil coating on a glass substrate with completely spreading FITC labeled BSA solution on

the superhydrophilic area and a water droplet sitting on the superhydrophobic area in a perfect

spherical shape (dimesion of glass substrate is 2cm*3cm). Corresponding schemes indicate

the complete wetting and non-wetting on the treated and untreated areas, respectively.

We produced well-defined superhydrophobic/superhydrophilic patterns with

different size and geometries such as square, circle and stripe by putting different

hollow mask layers onto the ormosil coatings during UV/O treatment. Dyed droplets

which were introduced individually to wetted patterns were well confined in the

desired spots and stripes thanks to the extreme wettability contrast between the treated

and untreated regions (Figure 3.6).

53

Figure 3.6: (a) Ormosil surfaces on glass substrate with dyed water solutions in square wetted

patterns with 1 mm edge dimension. (b) Ormosil surfaces on glass substrate with colored

aqueous solutions in stripe patterns with 1 mm thickness. (Blue –Methylene blue, Red-

Rhodamine 6G, Green- Mixture of Acridine orange and Methylene blue)

We demonstrated that high-throughput mixing can be performed by using droplet

arrays produced with different components as illustrated in Figure 3.7. Figure 3.8a

shows a high density of droplet (with diameters of 200 µm) arrays generated by rolling

a large droplet across the surface. Formation of droplet arrays were also recorded using

a high-speed camera with 2000 fps on the ormosil surfaces with patterns of 1 mm and

200 µm diameter. The distribution of equal volume of small droplets to the individual

spots from the rolling large droplet on the tilted was clearly observed. Low surface-

energy beyond the boundary of the high surface-energy patterns induced the separation

of small droplets.[52, 109] Furthermore, we can obtain flexible patterned surfaces by

coating ormosil surfaces onto flexible substrates such as cellulose acetate (Figure

3.8b). Even when surfaces were bent, they exhibit well defined regions of

superhydrophobicity and superhydrophilicity due to the hybrid nature of ormosil

networks that preserve their structure under mechanical stress.[17, 62] Since time-

dependent stability of extreme wetting contrasts on the patterned surfaces is desired

for long life screening platforms we examined contact angles of patterned ormosil

coatings after 5 month-storage period at ambient conditions. Patterned ormosil

coatings conserved their initial wettability performance confirmed with no significant

change in their WCA (Figure 3.9).

54

Figure 3.7: High-throughput mixing of individual droplets. (a) Arrays of droplets dyed with

different colors (blue-Methylene blue and red-Rhodamine 6G) were formed by individually

dispensing dyed water to the wetted spots on two separate patterned surfaces. Identical array

sizes (a 4x6 array on glass substrates with the same size) were used. Longer UV/O treatment

(~60 min.) was applied to the bottom surface compared to the top surface (~30 min.) to collect

most of the mixed solution on the bottom surface after mixing. (b) The patterned surfaces were

aligned using a microstage. Droplet arrays (c) during the contact and (d) after the contact. Each

individual droplet on the top surface mixed with its counterpart in the bottom surface. No

lateral mixing was observed between the droplets on the same surface demonstrating the

precise control of droplet behavior on the patterned surfaces. (e) Arrays of mixed droplets.

Most of the mixed solution remained on the bottom surface (left) due to the gravity and the

higher wettability of the patterned regions.

55

Figure 3.8: (a) High-density droplet arrays in circle patterns with 200 µm diameter. (b) A bent

cellulose acetate coated with ormosil and droplet arrays on the superhydrophilic patterns of

the ormosil.

Figure 3.9: Performance of 60 min-UV/O treated ormosil surface in wettability contrast after

storing for 5 months. Spherical droplet on the superhydrophobic region and Rhodamine 6G

aqueous solution on the superhydrophilic stripe patterns with corresponding WCA values.

56

3.5 Controlled protein adsorption and bacteria adhesion

The flexibility in designing arbitrarily shaped patterns with precisely controlled

wettability were exploited for controlling the biomolecular adsorption on the surfaces.

Circular spot patterns of 1 mm diameter were prepared on the surfaces with different

wettabilities by varying the UV/O exposure time. The patterned surfaces were then

soaked into the 1 mg/ml fluorescein isothiocyanate labeled bovine serum albumin

(FITC-BSA) solution to compare the biomolecular adsorption on the surfaces. Figure

3.10 shows fluorescent images taken from the BSA-adsorbed surfaces which were

treated for 15, 30, and 60 min, respectively by using excitation wavelength of 488 nm

for FITC. All the treated surfaces exhibited fluorescent signal on the patterns with no

signal on the untreated regions revealing the selective adsorption on the wetted

patterns. Interestingly, fluorescence intensity, i.e. concentration of FITC-BSA

increased with the increasing treatment time which was correlated with the contact

angle decrease (Figure 3.11a and 3.11b). In addition, diameter of the fluorescent spots

was slightly higher for 30 min- and 60 min-treated samples compared to the 15 min-

treated counterparts (Figure 3.10). These findings showed that the adsorbed protein

density as well as its spatial distribution can be controlled by adjusting the degree of

wettability of the patterns from hydrophilic to superhydrophilic.

57

Figure 3.10: Fluorescent images taken from the BSA-adsorbed surfaces treated for (a) 15 min,

(b) 30 min, and (c) 60 min using confocal microscopy. Samples were excited at 488 nm and

the emission was collected at 505 nm. Fluorescent signals denoted as green in the images

correspond to the emission of FITC in the wetted patterns whereas the black background

corresponds to the superhydrophobic regions with no fluorescent signal.

58

Figure 3.11: (a) Normalized fluorescence intensity of the wetted patterns with respect to UV/O

treatment time. (b) WCA values on the UV/O treated ormosil surfaces with respect to UV/O

treatment time.

Furthermore, we adsorbed GFP-expressing E. coli on the superhydrophilic patterns of

ormosil with square wetted patterns. Fluorescent signal of GFP which were only

observed in the wetted patterns shows the good confinement of the bacteria cells in

isolated microspots (see Figure 3.12).

59

Figure 3.12: (a) Fluorescent microscope image of GFP-expressing E. coli cells on the

superhydrophilic patterns of ormosil with square wetted patterns (1x1 mm) (b) Close-up view

of one wetted pattern with adhered bacteria. Fluorescent signal of GFP from individual

bacteria cells with ~2 µm dimensions were clearly observed.

60

Chapter 4

Conclusions

This thesis describes the design and the fabrication of organically modified silica

nanostructure based superhydrophobic coatings for practical applications. We focused

on increasing the stability of Cassie state of wetting on transparent superhydrophobic

ormosil coatings as a self-cleaning surface. In addition, we demonstrated

superhydrophilic patterned superhydrophobic ormosil surfaces as substrates for

biological and chemical screening.

For self-cleaning superhydrophobic surfaces, stability of the Cassie state wetting is

crucial. This thesis demonstrated a facile method to fabricate transparent and

superhydrophobic ormosil surfaces with robust Cassie states of wetting. We prepared

two superhydrophobic surfaces on glass substrates: (i) a single-layer microporous

coating and (ii) a double-layer coating with nanoporous bottom and microporous top

layers. The stability of the Cassie state of the coatings against the external pressure

was investigated with droplet compression and evaporation experiments. It was

observed that a droplet sitting on a single-layer coating can easily transform to a sticky

Wenzel state from a roll-off Cassie state under external pressures as low as

approximately 80 Pa. On the other hand, Cassie to Wenzel state transition was

observed at around 1600 Pa for a double-layer micro/ nanoporous coating, which is

almost 4-fold higher than the transition pressure for a Lotus leaf 27 (slightly above 400

Pa). The observed extreme stability of the Cassie state in a micro/ nanoporous coating

can be attributed to its double-layer porous structure. In addition, we proposed a

mechanism for extreme stability of the Cassie state in a double-layer coating.

61

We have additionally demonstrated a facile method to obtain hydrophilic patterns

on superhydrophobic ormosil coatings using simple UV/O treatment. When the

ormosil coatings are treated under UV/O, their superhydrophobic surfaces can be

converted to superhydrophilic through the replacement of methyl groups by the

hydroxyl groups with no change in their surface roughness. Treatment of the selected

spots on the ormosil surfaces leads to the formation of patterns with extreme

wettability contrasts. Arrays of droplets can be formed in desired sizes and shapes on

the treated surfaces using the introduced method and can be used to confine water-

dispersed substances on the chosen micro spots in a selective manner. We demonstrate

that biomolecules can adsorb on the wetted patterns and their concentration as well as

spatial distribution can be controlled with the UV/O treatment time. Such precise

control of biomolecule patterning, robustness and versatility of the patterned ormosil

surfaces on the flexible substrates present a value added for a variety of applications.

In summary, the outcomes of present work may provide researchers an adequate

ground for designing robust superhydrophobic surfaces for outdoor and underwater

applications where surfaces are exposed to high external pressures. In addition, simple

method for hydrophilic patterned ormosil surfaces with presicely controlled

bioadhesion may be useful for high-throughput screening substrates and facile

microfluidic test kits.

62

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